The electrodeless lamps with which the present invention is concerned generally comprise a lamp envelope containing a plasma-forming medium. The medium in the envelope is excited with microwave, RF or other electromagnetic energy, thereby generating a plasma which emits radiation in the ultraviolet, visible or infrared part of the spectrum. Important uses for such electrodeless lamps to date include curing coatings or inks by photopolymerization reactions, and semiconductor photolithography.
It is known that electrodeless lamps transfer a great deal of heat to the lamp envelopes during operation, and it has been found that the effectiveness with which the lamp envelopes are cooled limits overall lamp performance. Thus, the brightness with which energy is radiated by the lamp increases with the power density of the microwave or other energy in the lamp envelope; however, as the power density increases, so does envelope temperature, with a temperature being reached where the envelope melts if it is not adequately cooled. Therefore, the brightness which can be obtained from a lamp is ultimately a function of cooling. Also, in the case where a lamp is operating satisfactorily at a given envelope temperature, cooling the envelope to a lower temperature has the effect of substantially increasing bulb lifetime.
In one technique which has been used to cool electrodeless lamps, air is passed over a stationary lamp envelope. U.S. Pat. No. 4,042,850 describes one such method of cooling, which involves forcing air from a compressor into a lamp chamber over the lamp envelope. The power density achieved by this method of cooling is reported to be in the range of about 100 watts per cubic cm of lamp envelope volume. In an improvement described in U.S. Pat. No. 4,485,332, the lamp envelope is rotated while at least one stream of cooling gas is passed over the envelope surface. Using this method of cooling, a power density of about 500 watts per cubic cm. of lamp envelope volume has been obtained. However, even with this method, some hot spots remain which limit the average power density which can be achieved. At least a part of the non-uniform temperature is the result of uneven flow of cooling gas over the surface of the envelope.